1. Field of the Invention
This invention relates to a sheet feeding apparatus for feeding sheets (including original documents) in image processing systems such as a printer, a copying machine, and a stencil duplicating machine.
2. Discussion of the Background
FIG. 10 of the accompanying drawings shows a sheet feeding apparatus for a stencil duplicating machine. In operation, a sheet feed roller 202 (called xe2x80x9cthe feed roller 202xe2x80x9d) successively pays out sheets 201 from a sheet tray 200 from top to bottom. Each paid out sheet 201 is separated from the remaining sheets 201 by a separating roller 203 and a separating pad 204, and is fed into a space between a pair of register rollers 205. The register rollers 205 are rotated in synchronization with the rotation of a printing drum 206, so that an image perforated on a stencil wrapped around the printing drum 206 is transferred (or printed) onto the sheet 201. Thereafter, the sheet 201 is conveyed to a discharge tray 208 via an absorbing unit 207.
The feed roller 202 has at least a surface made of a material with a high friction coefficient such as rubber. A frictional force between the feed roller 202 and a top sheet 201 is designed to be larger than a frictional force between the sheets 201. The former frictional force depends upon a sheet feeding pressure (i.e. a pressure applied by the feed roller 202 to the sheets 201). The smaller the sheet feeding pressure, the oftener the feed roller 202 slips on the sheet 201, and fails to pay it out. Conversely, the larger the sheet feeding pressure (called the xe2x80x9cfeeding pressurexe2x80x9d), the oftener the feed roller 202 feeds a plurality of sheets 201 at a time.
Therefore, it is necessary to maintain the feeding pressure constant, since it is continuously variable with factors such as a kind, a size and an amount of sheets 201, a printing speed, and so on. Referring to FIG. 10, a sheet feeding pressure regulator 209 (called the xe2x80x9cfeeding pressure regulator 209xe2x80x9d) regulates the feeding pressure, and includes an arm 211 which is connected at its base to a stationary shaft 210, has a weight 212 at its free end, and is longitudinally movable at the free end. The arm 211 supports the feed roller 202 at the free end, so that the feed roller 202 is freely rotatable. The feeding pressure regulator 209 urges the arm 211 upward via a spring 214 connected to a side plate of the stencil duplicating machine via a regulating member 213. The regulating member 213 includes a plurality of hooks, with which the spring 214 is engaged so as to regulate the feeding pressure.
A pressure by which the sheets 201 comes into contacts with the separating roller 203 affects sheet separating functions. A separating pressure regulator 215 is provided in order to regulate this pressure, and comprises a spring 216 which is engaged with the separating pad 204 at its one end, and with a member 217 at the other end thereof. The member 217 or the separating pad 204 includes a plurality of hooks (not shown in FIG. 10) to which the spring .216 is secured so as to regulate a pressure applied by the separating pad 204 to the sheet separating roller 203, i.e. a sheet separating pressure (called the xe2x80x9cseparating pressurexe2x80x9d).
In the prior art shown in FIG. 10, in order to regulate the sheet feeding or separating pressure, it is necessary for an operator to manually change a position for securing the spring with a hook, which is very troublesome. Further, the sheet feeding or separating pressure delicately varies with kinds of sheets (e.g. thickness and quality of paper). Thus, this adjustment requires dexterity and quick response of the operator, and is not performed so reliably.
Japanese Patent Laid-Open Publication No. Sho 62-201736 proposed a sheet feeding mechanism which is intended to overcome the foregoing problems of troublesome manual operation and reduced reliability.
In this sheet feeding mechanism, when a sheet kind is input, a voltage which is associated with an optimum sheet feeding position for the specified sheet kind stored in a memory is compared with a voltage which is associated with an actual position of the sheets detected by a sheet feeding position detecting member. A hopper housing sheets is moved to its optimum position, and a feeding pressure, a separating pressure and rotation of a sheet feeding roller are controlled.
Specifically, as soon as initial conditions such as a size and kind of sheet and a printing speed are specified, the optimum sheet feeding data are retrieved from data stored in the memory.
In order to reliably feed each sheet to a printing section, the sheet feeding apparatus is continuously required to maintain appropriate sheet feeding and separating pressures in accordance with the kinds of sheets. However, it does not always follow that once the sheet feeding and separating pressures are appropriately determined in accordance with the kinds of sheets, neither no-sheet feeding nor multiple sheet feeding should take place.
This is because factors affecting the sheet feeding and separating pressures, i.e. the thickness and quality of sheets, tend to vary with environmental conditions in the stencil duplicating machine (e.g. temperature, humidity and so on).
The sheet feeding mechanism of the foregoing publication can overcome the problems caused by troublesome and reliable manual adjustment. However, when an optimum value is automatically determined in accordance with one initial condition, it is applied throughout the sheet feeding operation related to the initial condition. The sheet feeding mechanism is difficult to comply with conditions varying with time.
Therefore, it is very difficult for this sheet feeding mechanism to precisely and reliably prevent no-sheet feeding or multiple sheet feeding.
Further, the foregoing sheet feeding apparatus is disadvantageous in the following respect: no-sheet feeding caused by a worn feed roller; or waste of time when the operation is suspended in order to exchange a worn-out feed roller. At present, no particular measures have been taken from this viewpoint.
The present invention is intended to provide a sheet feeding apparatus which can automatically determine sheet feeding parameters such as sheet feeding and separating pressures, a sheet feeding time and so on in accordance with sheet feeding conditions such as a kind of sheet, a printing speed and so forth, adapt itself to varying sheet feeding conditions, and reliably minimize occurrences of no-sheet feeding and multiple sheet feeding.
Further, the invention is intended to provide a sheet feeding apparatus which can prevent problems caused by a sheet feed roller which wears with time.
In accordance with the invention, there is provided a sheet feeding apparatus for an image processing system which includes register rollers periodically conveying sheets from a sheet feed roller to a printing section. The sheet feeding apparatus mainly comprises a sheet feeding time sensor for detecting a sheet feeding time of a sheet, and a control unit. The sheet feeding time sensor is positioned upstream of the register rollers in a sheet feed path. The control unit compares the sheet feeding time detected by the sheet feeding time sensor with an experimentally predetermined reference sheet feeding time, and controls a revolution speed of the sheet feed roller on the basis of a compared result during or after a sheet feeding operation.
The sheet feeding apparatus continuously monitors a state of sheets which are being conveyed, and feeds sheets in an optimum state in response to any change in the sheet feeding conditions.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.